1. Field of the Invention
This invention relates to a monitoring device for lamp failure in airport navigation lighting, wherein the lamps are supplied via current transformers which are connected in series on the primary side to the secondary side of a high-voltage transformer which is connected on the primary side, via a constant-current regulator, to an AC voltage system. More particularly, the primary current of the current transformers is measured via a current measuring transformer and the voltage is measured via a voltage measuring transformer on the primary side of the high-voltage transformer and the output signals are fed to a monitoring device.
2. Description of the Prior Art
In airport landing-light systems it is necessary to monitor the lamps for failures due to broken helices, line breaks, or short circuits and to indicate the percentage of failed lamps. The voltage U.sub.K of the lamp circuit is given by EQU U.sub.K =Ri+L.sub.K di/dt.
The inductance L.sub.K includes here all the inductances of the circuit (such as the inductance of the high-voltage transformer, the inductance of the current transformer, and the line inductance). A low-cost way of making the measurement is based on the fact that at the instant of the current maximum, the derivative of the current with respect to time is di/dt=0, so that at this instant, the monitored circuit behaves like a purely resitive circuit. A low-cost measurement is therefore reduced to the determination of current and voltage to ascertain the ohmic resistance in the circuit at the instant of the current maximum. For a given brightness level, this ohmic resistance is a constant value, since the r.m.s. value of the current is kept constant by means of the constant-current regulator arranged in the primary circuit of the high-voltage transformer. Therefore, by measuring the voltage and the current at the instant of current extreme, the resistance of the monitored circuit and any change in this value can be determined. Conclusions as to the percentage of failed lamps can be drawn from a change in this value.
In a commercially available monitoring device, the output signals of the current measuring transformer and of the voltage measuring transformer are each fed to a proportional amplifier and subtracted from each other at a summing point. A signal voltage is generated from the output signal of the proportional amplifier associated with the current measuring transformer which is fed, together with the voltage produced at the summing point, via a peak-value former, a pulse-width control, and a signal voltage generator, to an analog switch. The analog switch is always switched into conduction for the duration of the signal voltage, so that the voltage present at the summing point can be fed to a time-delay stage following the analog switch. The output signal of the time-delay stage is fed to a limit indicator and to an indicating instrument connected in parallel thereto so that the number of failed lamps can be determined and a protective measure initiated if a certain maximum number is exceeded. The output signal of the time-delay stage is a measure of the number of failed lamps, i.e., the value of the output signal is zero if the load circuit is intact. Through cooperation of the peak-value former, the pulse width control and the signal voltage generator, a pulse-shaped voltage, having a pulse width which is inversely proportional to the maximum value of the current, is generated at the instant of each current maximum.
This known monitoring device gives reliable indications for only one brightness level and requires very elaborate adjustments.
It is an object of the invention to develop a monitoring device of the type mentioned at the outset which requires less adjusting effort while employing a simplified circuit and which can easily be used with different brightness levels due to the simple adjustment requirements.